Spread spectrum signals are used in digital radio systems for telecommunication and navigation purposes. In navigation systems (Global Positioning System, GLONASS, GPS/GLONASS), a receiver processes several spread spectrum signals, each one transmitted by a different satellite, to track the distance of the receiver from each satellite, and thereby, to determine its own position. In telecommunication systems, spread spectrum CDMA signals are used for (i) combating interference, (ii) transmitting at very low power to avoid detection/interception and (iii) multiplexing one channel over many users. This signal format is becoming the standard for wideband wireless communications.
Spread spectrum CDMA signal processing is characterized by expanding the bandwidth of the transmitted signal by a large factor (typically higher than 100) through pseudorandom noise (PN) modulation, and by compressing the bandwidth of the received signal by the same factor. PN modulation is implemented by two techniques: (i) by transmitting a PN sequence of binary pulses in each data bit interval, which is referred to as a direct sequence/spread spectrum (DS/SS) system, and (ii) by employing different carrier frequencies in each data bit interval (or fraction thereof) so that the record of used carrier frequencies constitutes a PN sequence, which is referred to as a frequency hopped/spread spectrum (FH/SS) system. In both methods, bandwidth compression is accomplished by correlating the received signal with the known PN chip or carrier frequency sequence. Due to spread spectrum signal processing, every incident interference is spread at the receiver over the whole system bandwidth, whereas the bandwidth of the desired signal is compressed. As a result, the effective interference power is smaller than the total incident interference power by a factor equal to the bandwidth expansion factor. For the above reason, the bandwidth expansion factor is referred to as the processing gain of the spread spectrum system.
In a DS/SS receiver, such as GPS, it is possible to suppress a narrowband interference beyond the processing gain, by filtering the received signal prior to despreading (also referred to as precorrelation) through a narrow band filter such as an adaptive temporal filter (ATF). A digital implementation of an ATF is the subject of existing patents, e.g., U.S. Pat. Nos. 5,268,927 and 5,596,600 incorporated herein by this reference. Other approaches for narrowband interference cancellation have used operations in the frequency domain, as evidenced by U.S. Pat. No. 5,410,750, which is are incorporated herein by this reference. These methods are not very effective, however, against wideband interference. In this disclosure, references to "adaptive temporal filter" or "ATF" are properly interpreted as including these methods, or any other self-contained approaches to narrowband interference cancellation.
The spread spectrum format of the GPS or other wideband signal also provides a degree of protection against wideband interference. But, even with these features, it is possible and practical for an adversary to deny spread spectrum CDMA signal reception in a geographic area by fielding a large number of wideband interferences or a mix of both wideband and narrowband interferences. Directional antennas, using multi-element arrays, have been used to provide the required protection. Several spatial (directional) interference cancellation techniques employing multi-element array antennas have been applied to combat wideband and narrowband interferences. In an adaptive array antenna (more generally referred to as spatial filters or smart antennas) antenna element outputs are multiplied by controlling weights to steer and shape the antenna array pattern to either direct nulls towards the jammers, direct a beam towards the desired signal, or form an antenna pattern that accomplishes both by optimizing the signal-to-interference-plus-noise (SINR) power ratio. An antenna weight control algorithm appropriate to the criterion is selected to produce the controlling weights. Application of adaptive antennae, i.e., the spatial filter, for interference suppression by forming antenna pattern nulls in the directions of interferers is existing art. The antenna controlling weights, to form the desired antenna pattern, can be applied at RF, at IF or at digital base band outputs of the antenna elements. In an analog implementation, the weights are applied at RF/IF, and are customarily represented as phase shift and attenuation. Limitations of analog electronic implementations for weight application include limited resolution in gain and phase control and imperfections including nonlinearity, temperature sensitivity and aging. In a digital implementation, the antenna element outputs are appropriately downconverted, filtered and digitized at or near base band in a signal conditioning module, and the controlling weights are applied digitally and represented in terms of in-phase (I) and quadrature (Q) weights. Application of the antenna controlling weights to produce the desired antenna array pattern, in this case, corresponds to digital complex multiplication and summation operations. Digital implementations of adaptive array antennae are the subject of existing patents. See, for example, U.S. Pat. No. 5,694,416, incorporated herein by this reference.
Regardless of the adaptive antenna implementation approach, the effectiveness of the adaptive antennae are constrained by the number of elements (N) in the array which controls the available degrees of freedom to form nulls towards the interferers. An N-element antenna array has N-1 degrees of freedom and, therefore, can be effective in forming only up to N-1 independent nulls. Since the size of the antenna array and the associated electronics is directly related to the number of elements, and since in most practical applications size, power, weight, and cost are at premium, increasing the number of antenna elements to deal with a larger number of interferers is costly because of the cost of the antenna element itself and the cost of electronics which includes signal conditioning subsystems associated with each element of the antenna pattern former. Most adaptive antennas have used between two and seven antenna elements depending on the application. Precision guided munitions applications tend to use fewer elements, typically three or four, because of their limited size. Aircraft applications, on the other hand, can afford to use somewhat larger arrays and the associated electronics.
It is possible to add degrees of freedom to an array antenna, a process which is effective against narrowband interference. Essentially, a delay line is added in each element channel so as to make available not only the current signal out of each element but also a number of delayed signals. The beam is then formed by summing, with appropriate weights, all of the current and delayed signals from all of the elements. If there are N antenna elements and M delays (including the zero delay) in each channel, the beam is formed from a total of N.multidot.M signals and the system controller must determine N.multidot.M weights which requires a computational process of an order of N.multidot.M or higher. This is referred to as Spatial-Temporal Adaptive Processing (STAP). The required processor power, however, becomes very burdensome even for modest values of N and M.
By contrast, if a spatial filter (order N) and a temporal filter (order M) can be implemented separately (order N+M), but integrated in an effective manner, the processing burden is much smaller. This process offers a more efficient means of dealing with larger fields of interferers consisting of a mix of both narrowband and wideband types than either increasing the number of antenna elements or introducing STAP processing. Until now, however, such a method has not been implemented.
Therefore, in summary, in the prior art, attempts have been made to combine spatial and temporal filters. Spatial filters, as discussed above, such as an adaptive antenna array system with N array elements, however, normally operate to receive a spread spectrum signal and to null N-1 interference signals based on signal strength and not whether the interference signal is wideband or narrowband. If, however, there are more than N-1 sources of interference and any one of these sources outputs a narrowband signal stronger than the weakest wideband interference signal, the spatial filter will inefficiently null the stronger narrowband interference signals instead of the weaker wideband interference signals. Since the temporal filter is ineffective against these wideband interference signals, they interfere with the receiver and render it unable to interpret the spread spectrum signal.
Thus, the previous attempts at combining the spatial filter with the temporal filter, which consisted of cascading the two filters--spatial filter followed by the temporal filter, have been unsuccessful and have been reported to fail to improve interference suppression performance further than what is achievable by the spatial filter alone.